Fibers used as a carrier of catalytic material, present in catalytic converters are well known in the art.
Fibers, usually ceramic fibers, but also metal fibers, are transformed into a fiber volume, in which fibers are mechanically bonded to each other. Such fiber volumes are then coated with the catalytic material or materials necessary to provide the catalytic conversion for which the catalytic converter is to be used.
Usually, the fibers are first bonded to each other, using sintering or using a bonding agent, and the obtained fiber volume is immersed into a solution of catalytic material. Possibly, the bonded fibers are already present in the canning of the catalytic converter.
As an example, U.S. Pat. No. 4,360,957 describes an exhaust gas treating unit, which comprises fibers having a mechanical bond to each other due to the use of a bonding agent which already comprises the catalytic material.
As a general disadvantage, the fiber volume may not be coated equally over the whole depth of the filter volume. Also the canning inner surface may be coated with catalytic material, which may have only a limited or no catalytic working when the converter will be used. Further, during use, the mechanically bonded fiber volume will be subjected to relatively severe thermal tension. Most catalytic reactions are exothermic and generate heat. Due to this temperature increase, the fiber volume will tend to expand, which in its turn may cause locally breakage of the mechanical bonds between fibers, resulting in so called cracks in the mechanical fiber structure These creates preferential paths via which the reactants, such as gasses or liquids, will flow. The latter may cause the catalytic reaction to take place incomplete or even not to take place at all, due to insufficient retention time, being the time a reactant needs to pass through the catalytic converter. Possibly, if the temperature becomes too high, the fiber volume may melt down.
As the fiber volume with preset boundaries is made prior to installing in the canning of the converter, small tolerances between fiber volume and canning are required. These small tolerances are also needed to avoid preferential reactant flows near the canning, causing decreasing retention time of the reactants at the surfaces of the fibers where the catalyst material is present.
As an other drawback, the shapes of the canning are restricted, as the canning must have an opening, via which the fiber volume as a whole must enter the inner volume of the canning.